Standby external rate control and implanted standby heart pacer

ABSTRACT

The pacing pulse rate of a body implanted standby heart pacer is selectively controlled with a remote rate control device. The pacer has a coil which transmits a signal to the device whenever a natural heartbeat occurs or when an artificial stimulus is delivered to the heart. The same coil receives signals from the remote control device, and, if these signals are faster than the set rate of the pacer, the remote rate control will establish the rate of the pacer. The pacer paces the heart, when it requires artificial stimulation, at whatever rate the remote control is set. Signals radiated from the pacer are sensed by the remote rate control and are used to inhibit the remote transmitter for a specific period after each natural beat so there can be no competition between natural and artificial heart stimuli.

IJnited States Patent 1 Bowers [54] STANDBY EXTERNAL RATE CONTROL ANDIMPLANTED STANDBY HEART PACER [75] Inventor: David L. Bowers, MilwaukeeCounty, Wis.

[73] Assignee: General Electric Company [22] Filed: Oct. 19, 1970 [21]Appl. No.: 81,846

[52] US. Cl. ..128/419 P, 128/422 [51] Int. Cl. ..A6ln U236 [58] Fieldof Search ..l28/4l9 P, 421, 422

[56] References Cited UNITED STATES PATENTS 3,528,428 9/1970 Berkevits..l28/419 P 3,426,748 2/1969 Bowers ..l28/4l9 P 3,241,556 3/1966 Zacovto..128/419 P OTHER PUBLICATIONS Holcomb et al. Medical & BiologicalEngineering, Vol. 7, No. 5, September, 1969, pp. 493-499 51 Feb. 20,1973 Primary Examiner-William E. Kamm Att0rney-Frank L. Neuhauser, OscarB. Waddell, Joseph B. Forman, Jon Carl Gealow and Arthur V. Puccini [57]ABSTRACT The pacing pulse rate of a body implanted standby heart paceris selectively controlled with a remote rate control device. The pacerhas a coil which transmits a signal to the device whenever a naturalheartbeat occurs or when an artificial stimulus is delivered to theheart. The same coil receives signals from the remote control device,and, if these signals are faster than the set rate of the pacer, theremote rate control will establish the rate of the pacer. The pacerpaces the heart, when it requires artificial stimulation, at whateverrate the remote control is set. Signals radiated from the pacer aresensed by the remote rate control and are used to inhibit the remotetransmitter for a specific period after each natural beat so there canbe no competition between natural and artificial heart stimuli.

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DAVID L. BOWERS Attorney STANDBY EXTERNAL RATE CONTROL AND IMPLANTEDSTANDBY HEART PACER BACKGROUND OF THE INVENTION stimuli whichoccasionally reappear in some patients. 1

The demand type pacer was then developed. This type of pacer deliveredone or more stimulating pulses to the heart only if a natural pulse wasmissed or delayed for a physiologically unacceptable period of time. Adisadvantage of the demand type pacer is that it never lets the heartrate decline below the set minimum .rate of the pacer. The chosen ratewas usually high enough to allow the heart to fulfill metabolic demandsincidental to fairly vigorous physical activity. The same rate wasusually too high for a patient Who desired to rest or sleep.

Standby pacers were then developed to overcome the above disadvantages.This type of pacer is similar to the demand type in respect to itssupplying an artificial stimulating pulse if the natural stimulatingpulse of the heart is missed or unduly delayed. However, the standbypacer has hysteresis which means that it permits the heart rate to dropthrough a range which corresponds with low physical activity before anyartificial pace pulse is supplied. When the heart rate drops below apredetermined minimum, the standby pacer supplies one or morestimulating pulses as required at a higher rate which is commensuratewith fairly vigorous physical activity.

Sometimes during the course of managing patients with implanted pacersphysicians desire to elevate the heart rate above the intrinsic rate ofthe implanted pacer regardless of whether it is a fixed rate, demand orstandby type. For instance, a high heart rate is often desired when thepatient is convalescing from other illnesses and when heart functiontests are being performed.

Early in the history of pacer development, one type of fixed rate pacerwas adapted for having its intrinsic pace pulse rate increased by meansof a control device which was located outside Of the body. The implantedpacer included a coil in which a voltage could be induced byelectromagnetic radiation from the external control. The external deviceincluded a transmitting coil which was driven by an external pulsegenerator. If the external device was set for a faster rate than theintrinsic rate of the pacer, the former captured the pacer and drove itat the set rate of the external device. The induced voltage pulses inthe coil of the implanted pacer affected a biasing circuit and causedthepulse generator of the pacer to pulse earlier than was the case whenit was controlled exclusively by its own timing circuit.

Heretofore there has been no way to increase the rate of an implanteddemand or standby pacer without sacrificing the demand or standbyfeatures. The prior art type of control for increasing the rate of afixed rate pacer increased the rate at all times. Thus, competitiondeveloped when the heart interjected one or more natural beats. This isuncomfortable to the patient and has some adverse physiological effectswhich are well known.

An object of the present invention is to provide a rate control devicewhich may be implanted in the body or located externally of the body tochange the intrinsic rate of an implanted demand or standby cardiacpacer.

A further object is to provide a remote rate control which is inhibitedfor a predetermined period of time following occurrence Of a naturalheartbeat so that there will be no competition between remotely inducedartificial stimuli and natural stimuli.

A still further object is to provide a remote pacer control Whichpermits hysteresis or delay following a natural beat so that the heartmay operate through a range of rates below the high set rate before theheart is artificially stimulated by the remote device.

Briefly stated, one embodiment of the invention requires that theimplantable pacer be modified so that it radiates an electromagnet pulsefor every natural heartbeat and every pacer stimulated beat. The newremote rate control also radiates control pulses to the pacer to makethe pacer follow the rate set by the remote control. The remote ratecontrol device has a coil which receives pulses from the pacer andproduces corresponding signals which cause the remote pulse generator tobe inhibited for a predetermined period after occurrence of a heartbeat.The delay or hysteresis period may be held constant or be caused to varyin proportion to the rate setting of the remote control.

How the above stated general objects and other more specific objects areachieved will appear from time to time throughout the course of ensuingmore detailed description of embodiments of the invention taken inconjunction with the drawings. In this description and in the claims theterm standby pacer will be used interchangeably with and equivalent todemand pacer since the invention is applicable to both regardless ofnomenclature.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of animplantable standby type cardiac pacer incorporating the invention;

FIG. 2 is acircuit diagram for one type of standby pacer remote ratecontrol;

FIG. 3 is a schematic diagram of an alternative embodiment Of anexternal control for an implanted standby pacer;

FIG. 4 shows some waveforms in respect to time relative to aconventional standby pacer; and

FIG. 5 shows some waveforms with respect to time relative to a standbypacer that is controlled by the new control device. I

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. shows a standby pacer whichhas been modified for having its pacing rate controlled externally. Inthis figure one may see a diagram of a heart 10 which is connected bymeans Of a pair of insulated leads 11 to the output terminals 12 and 13of the standby pacer. Leads 11 serve the dual purpose of deliveringartificial pacing pulses to the heart and detecting the presence ofnatural heart signals. Except for leads 11, the circuit elements shownin this figure are usually encapsulated in a solid epoxy resin andcoated with body tolerant material, such as silicone rubber.

This type of pacer is adapted for delivering stimulating pulses to theheart only if natural beats are delayed or missed. Hence, means areprovided for sensing the occurrence of natural beats and inhibiting theartificial pulse generator. A line 14 therefore runs from heart terminal12 to a sensing preamplifier and filter 15 which is shown in block format the front end of the pacer. There will be an explanation later on howthe natural heart signals which are delivered to preamplifier 15 fromthe heart over conductor 14 are Processed to control a pulse generatorcomprising transistors Q4 and Q5.

Preamplifier and filter l constitute a heart signal detecting means. Thefilter has a band-pass of between 20 and 40 Hz. Each natural orartificially induced heart signal is rich in fundamental frequencieswithin this band-pass region. When a heart signal or R-wave is received,filter 15 is shocked and produces a ringing output signal of the formmarked 16 on the output side of the filter. An alternating ringingsignal 16 is produced by the filter regardless of whether the incomingR-wave signal from the heart is positive or negative.

The ringing signal is, a-c coupled through a capacitor C1 to a gatedthreshold trigger 17 which is shown in block form. The trigger is biasedby a voltage divider comprising resistors R1 and R2. For any positiveswing of ringing signal 16, there is one output pulse such as 18delivered from the trigger 17. The output pulse appears at the top ofresistor R4 every time a natural heartwave occurs. Trigger 17 isinhibited and produces no output signal when an artificial stimulatingor pacing pulse is being delivered to the heart. Inhibition or gating ofthe signals is accomplished with a transistor gate Q1, as will bedescribed later.

Pulses 18 occurring at the rate at which R-waves are being detected onthe heart are delivered to an interference rejector 19 Which is shown inblock form. For present purposes the details Of interference rejector 19are not pertinent. it is sufficient to be aware that rejector 19 issimilar to a low pass repetition rate filter which yields an outputsignal for input pulses associated With normal heart activity andexcludes high repetition rate signals associated with certain types ofinterference such as electric razors, electric drills and automobileignition systems. The rejector is not frequency responsive but is pulserate responsive. For present purposes it is sufficient to know thatinput pulses 18 Of enough amplitude and at a low enough rate willproduce an output pulse 20.

Output pulses 20 are delivered to the base of a transistor O2 whichconducts for the duration of every pulse 20. When transistor Q2conducts, it resets an hysteresis. circuit which will be described.Before discussing the hysteresis circuit, the artificial pacing pulsetiming circuit and the circuit which it drives will be discussed.Attention is invited to the right side of FIG. 1 where one may see aheart coupling capacitor C7. This capacitor is connected to the positiveside of batteries 21 through a high value charging resistor R18. Thetime constant of R18 and C7 is such that the capacitor will charge to asufficient voltage level for stimulating the heart during the intervalbetween beats. C7 is charged from batteries 21 through a series circuitincluding R18, C7, conductor 14, the heart 10 and back to the negativeside of batteries 21. When the heart demands an artificial beat,transistor O6 is rendered conductive so as to quickly dischargecapacitor C7 through the heart to thereby stimulate it. The

discharge path originates with the positive side of C7 and continuesthrough the series path including Q6, R17, a coil L1, the heart 10 andback to the negative side of C7 on conductor 14. The slow charge rate ofC7 results in a current that is insufficient to stimulate the heartswhenswitching transistor O6 is turned on, however, comparatively heavycurrent flows from capacitor C7 and the heart is stimulated. Thestimulating pulse width is about 1 to 2 milliseconds. Each time astimulating pulse is delivered, coil L1 transmits an electromagneticpulse to outside of the patients body. C6 is used to attenuate highfrequency interference signals.

Switching transistor O6 is controlled by a timing pulse generatorincluding transistors 04 and Q5. The timing pulse generator has a timingcircuit including resistors R10 and R11, C3, diodes D2-D4, and resistorR13. The diodes and R13 are also paralleled by coil L1. The junctionpoint of R13 and L1 is connected to the negative side of the batteries21. If no artificial stimulating pulses are demanded by the heart, C3will charge to a predetermined voltage level near battery voltage andwill ordinarily remain at that level.

One side of C3 is connected to the emitter of a transistor Q4 whose baseconnects through resistor R12 to point A, which is in a voltage dividerincluding resistors R8 and R9. The ratio of these resistors is such thatpoint A is held more positive than the top of capacitor C3 in which casethe emitter-base circuit O4 is not forward biased. However, for reasonswhich will be explained, when a natural heartbeat is missed or delayedfor a specified period of time, point A will go more negative in whichcase capacitor C3 begins discharging through the emitter-base circuit ofQ4 and turns the latter on. This also renders the emitter-to-collectorcircuit of transistor Q4 conductive and causes a pulse 22 to appear onits collector. The pulse duration is about 2 milliseconds which isdesirable for stimulating the heart. The latching voltage keeping thismultivibrator circuit in conduction appears across resistor R12 and isadditive to the bias voltage across resistor R8.

When Q4 conducts, a voltage appears at the top of its collector resistorR14 and this voltage is applied through pulse width control resistor R16to the base of O which causes the heart coupling capacitor C7 todischarge and stimulate the heart. When the stimulus pulse terminates,the timing capacitor C3 recharges and therefore repeats the timing cycleif no natural beat appears within a predetermined time period.

When 04 conducts Q5 also conducts and the potential appearing on itsemitter is applied by way of a conductor 23 and a resistor R3 to thebase of the inhibiting transistor Q1 at the far left of the drawing.When Ql conducts, the input of threshold trigger 17 is pulled down toground potential in which case it cannot conduct and, hence does notsense nor respond to artificial pacing signals.

A filter network including capacitor C5 and resistor R15 prevent theemitter of transistor Q5 from floating above ground potential while atthe same time providing emitter impedance during the conductive state ofQ5.

Attention is now invited to the manner in which point A is divider R8and R9 has its voltage state selectively changed in accordance withwhether or not an artificial pacing pulse is demanded by the heart. Thisis accomplished by an hysteresis circuit which includes capacitor C2.The charging circuit for C2 is a series circuit starting at the positiveside of battery 21 and continuing through diode D1 and transistor Q2 tothe negative side of the battery. Diode D1 is forward biased when C2 ischarging. As explained earlier, whenever a natural heartbeat occurs, Q2conducts in which case capacitor C2 is charged rapidly. C2 is connectedacross the emitter and base circuit of a transistor Q3 in the hysteresiscircuit. The collector of O3 is connected to point A and its emitter iseffectively connected to the top of R8 or positive side of battery 21 sothat when Q3 is conducting R8 is effectively short circuited and hasminimum voltage drop across it which is the emitter to collectorsaturation voltage of Q3. Point A is then near positive battery voltage.For a period after a natural heartbeat, C2 keeps Q3 forward biased byreason of a voltage drop occurring across discharge resistors R6 and R7.C2 must discharge through R6, R7 and the emitter of Q3 since D1 is nowreverse biased. As long as Q3 conducts, point A is held relativelypositive since there is minimum voltage drop across R8. If C2 dischargesthrough R6 and R7 to a certain low level, hysteresis transistor Q3 willturn off, causing voltage to appear across R8 and a drop in thepotential at point A. This will permit timing capacitor C3 to forwardbias Q4 and pulse the heart as described heretofore. As long as Q3remains nonconductive, timing capacitor C3 will repetitively charge anddischarge with a period which is shorter than the discharge period of C2in the hysteresis circuit. The result is that after the initial delaycaused by the hysteresis circuit, Q4 will be rendered conductive at ahigher rate than the rate of the hysteresis circuit.

The normal hysteresis mode of operation of the implanted pacer may bevisualized by referring to FIG. 4 which shows the time relationship ofactual and artificial heart stimuli waveforms. Assume, for instance,that a succession of electrocardiograph waves having R- wave peaks 25have been occurring naturally. Assume also that maintenance of desirednatural rhythm would require existence of a natural signal at 26, butthat this natural signal does not occur or is delayed for anunacceptable period. The hysteresis period will then elapse after whichan artificial stimulating pulse 27 will be applied to the heart. Thehysteresis period is indicated as being one second, corresponding with a60 pulses per minute rate. However, pacers with a longer or shorterhysteresis period are available on order depending on what the physicianspecifies as being the desired minimum heart rate for the patient.

In FIG. 4 one may see that unless a natural heartwave occurs within apredetermined period of time after artificial pulse 27, anotherartificial pulse 28 will be supplied within a period that is shorterthan the hysteresis period. In this case, the second and ensuingartificial pulses are indicated as being supplied within 0.857 secondswhich corresponds with 70 pulses per minute. Whenever a naturalheartwave occurs, the hysteresis circuit is reset and an artificialpulse will not be supplied until expiration of the hysteresis period.Thus, the heart may drop down to a low rate before an artificial pulseis supplied and thereafter any required artificial pulses are deliveredon a shorter period or higher rate.

The means for transmitting a signal indicative of the occurrence of anatural heartwave from the implanted pacer to outside of the body willnow be described in reference to FIG. 1. For this purpose, the pacer isprovided with a capacitor C4 which charges in a time that is shortcompared with the time between natural or artificially stimulatedheartbeats. The charging path for C4 starts at the positive terminal ofthe battery and includes a series circuit comprising R5, C4, coil L1,and back to the negative terminal of the batteries 21. The charging timeconstant is short compared with the time between either natural orartificially stimulated beats. As explained earlier, whenever a naturalbeat occurs, transistor Q2 conducts to reset the hysteresis circuit. Atthe same time capacitor C4 discharges through transistor Q2 and coil Llwhich are all in a series circuit. The discharge wave form at the coilL1 radiates an electromagnetic pulse to the outside of the patientcorresponding with every natural heart signal. If there is no naturalbeat, Q2 does not conduct and coil Ll does not transmit unless anartificial pulse occurs which will produce a signal across L1 aspreviously explained.

The purpose of the new external rate control which will be described isto increase the rate of the implanted pacer so that the heart beats atrate different than that dictated by either the hysteresis circuit orthe intrinsic pulse rate of the pulse timing generator which includestransistors Q4 and Q5. It will appear subsequently that the rate of theimplanted pacer is charged by inducing electromagnetic pulses into coilL1 from the external control unit. Any time the external rate control isset higher than the intrinsic rate of the implanted pacer, the externalcontrol will capture or dominate the internal unit.

When coil Ll receives electromagnetic pulses from the external ratecontrol, a voltage is developed across the coil by transformer action.The same voltage is developed across series connected diodes D2 D4 andR13 which are effectively connected across coil L1. Thus, a receivedpulse causes the anode of diode D2 to rise in potential and thispotential is additive to the potential on capacitor C3 which is inseries with the diodes. This added voltage is sufficient to forward biasthe emitter of timing pulse generator transistor Q4 so as to render thistransistor conductive earlier than would have been the case if thevoltage on C3 were relied I upon exclusively to forward bias Q4. Theadded voltage is sufficient to forward bias Q4 if C3 is fully chargedeven though point A is relatively positive due to transistor Q3 of thehysteresis circuit still causing R8 to be bypassed. Under anycircumstances, then, Q4 may be rendered conductive by the voltage pulsesproduced across coil L1 and the implanted unit will be caused tostimulate the heart at whatever rate the external control is set for.

One embodiment of an external rate control is depicted in FIG. 2. Thisrate control has its own hysteresis, that is, it not only dominates theintrinsic pulse rate of the implanted pacer but it also retains anindependent hysteresis feature even though it is operating at a higherset rate. The mode of operation may be understood by referring to FIG. 5which shows the time relationship between natural and artificial pulseswhen the external control unit is applied to the implanted pacer. Assumefirst that the intrinsic pulse rate of the implanted pacer is seventypulses per minute (p.p.m.), for example, and that it is desired to stepup its rate to ninety pulses per minute, for instance by use of theexternal control. Assume that a natural R-wave 30 has occurred asindicated. This will be followed by an hysteresis period, set by theexternal control unit, which is 0.750 seconds in this example,corresponding with an artificially induced pulse rate of eighty pulsesper minute. At the end of the hysteresis period, an artificial pulse 31is induced. If this pulse is not followed by a natural heart wave withina predetermined time, the next artificial pulse 32 will occur in 0.667seconds which corresponds with ninety pulses or heartbeats per minute.Ensuing pulses will occur at this short period until the next naturalheartbeat occurs at which time the hysteresis circuit is reset. In someapplications it is desirable to have the hysteresis period equal to theartificial pulse period. To recapitulate, the external controlstimulates the heart at a selected rate which is different than butusually selected higher than the intrinsic rate of the implanted pacerand the external unit also permits the heart rate to drop below the highrate of the external control but usually not less than the hysteresisrate of the implanted pacer before any artificial pulse is induced fromoutside the patient.

The external control shown in FIG. 2 has some of the characteristics ofthe standby implanted pacer which has been described above. A notableexception is that the external unit delivers control pulses to atransmitting coil L2 which may be placed on the body in proximity withthe implanted pacer. The input of the external unit also has a pulsereceiving coil L3. Coils L2 and L3 may be flat coils with the detectingcoil L3 inside and concentric with the transmitting coil L2 and they maybe included in a single package. The detecting coil L3 preferably has aferrite core for improving the flux density that results fromtransmission of electromagnetic pulses from coil L1 in the implantedpacer due to either the discharge of capacitor C4 when a naturalheartbeat occurs or during the delivery of an internal stimulus pulse.

In the external control shown in FIG. 2, the detector which receivespulses from the internal pacer includes coil L3 and a sensingpreamplifier 35. For every detected pulse, there is a ringing outputpulse 36 which is a-c coupled through a capacitor C8 to a gate 37. Thegate is biased by a voltage divider comprising resistors R20 and R21.When the gate 37 is gated with an input pulse 36 it produces an outputpulse 38 at the top of R22. These pulses 38 are supplied to the bases ofthe Darlington transistor pair Q8 and Q9, turning them on. The basebiasing resistor R23 for transistor Q9 is in parallel with a filtercapacitor C9. The collectors of the two transistors are connectedtogether and to a common collector resistor R24. Thus, each timeimplanted pacer coil L1 transmits a pulse that corresponds with eitherthe occurrence of a natural heart wave or a stimulated internallygenerated heart wave, external coil L3 receives the transmitted signaland transistors 08 and Q9 conduct. It will be explained subsequentlythat Q8 and Q9 do not conduct due to gate 37 being inhibited when theexternal control generates a pulse which is transmitted to the implantedpacer.

The external control pulse timing circuit in FIG. 2 includes transistorsQ10 and 011. The base of Q10 and the collector of Q11 are connected to apoint B of a voltage divider circuit comprising resistors R25 and R26.R26 is shunted by a filter capacitor C10. The conductor betweenresistors Q10 and Q11 and point B includes a resistor R27 which performsthe same function as R12 in FIG. 1.

The pulse period for the external control is established with a timingcircuit including a fixed resistor R28 in series with an adjustableresistor R29 and a timing capacitor C11. When the external control inon, C11 charges from batteries 39 through resistors R29 and R28 and backto the negative side of the battery. When C1 1 has charged to a voltagewhich is in excess of the sum of the forward biasing voltage at point Band the diode drop of the emitter-base circuit of Q10, the latter willbecome forward biased and will conduct through its emitter to collectorpath and produce a pulse which appears on the top of R31. The forwardbiased transistor Q11 operates regeneratively with Q10 and turns on thelatter very hard. Q11 has an emitter resistor R30 in parallel with afilter capacitor C12.

Square wave pulses which appear on collector resistor R3] of transistorQ10 are furnished through a resistor R32 to the base of drivingtransistor Q12. The value of R32 determines the width of the pulseapplied to Q12. A capacitor C13 is connected for being chargedcomparatively slowly through a resistor R33 which connects directly tobattery 39. The charging path for C13 is completed through a resistor 34to the negative side of battery 39. C13 charges slowly to near batteryvoltage between pulses and is discharged rapidly when Q12 is renderedconductive. The discharge path of C13 is through Q12, its emitterresistor R35, and the input terminals 39 and 40 of a driving amplifier41 which is shunted by a resistor R34. The output from driving amplifier41 consists of pulses which are delivered to transmitter coil L2. Thesepulses are electromagnetically coupled to coil L1 in the implanted unit.

When a control pulse is transmitted from coil L2 to the implanted pacer,the latter will deliver a stimulating pulse to the heart. This willresult in an electromagnetic pulse being radiated by implanted coil L1which pulse will be sensed by external coil L3. If this fedback signalwere processed in a manner that would cause external transistorsQS andQ9 to conduct, the timing of the external pulse generator would beaffected by reason of C11 being wholly or partially discharged. Hence,when the external control produces a pulse, gate 37 is inhibited. Thisis done by applying the pulse signal by way of conductor 28 and R38 tothe base of inhibiting transistor Q13 to make it conduct for the pulseduration. Conduction of Q13 effectively grounds the input of gate 37 sothat it will not pass a signal which would make Q8 and Q9 conduct. Thesetransistors should conduct only when implanted coil Ll radiates a signalcorresponding with natural heart signals or internally artificiallygenerated heart stimulating signals.

Going back to the pulse timing circuit, it will be seen that adjustabletiming resistor R29 is ganged to another adjustable resistor R36 whichis connected to C11 and as a diode D5 in its circuit, the cathode ofwhich connects to the collectors of transistors Q8 and Q9. As explained,when receiver coil L3 detects a pulse from the implanted pacer, Q8 andQ9 are rendered conductive in which case Cl 1 discharges before itsvoltage reaches the level which would allow forward biasing of Q10 andthe production of a driving pulse. The discharge path for thisanticipatory discharge of C11 is from the capacitor through adjustableR36, diode D5 and the collector-to-emitter paths of Q8 and Q9.

The voltage that remains on C11 when it is prematurely dischargeddepends on the set resistance value of R36. lf R36 is set to zero ohms,only the impedances of D5, Q8 and Q9 will limit current flow in whichcase C11 will discharge substantially completely. In this case a longertime will be required to recharge C11 and there will be some hysteresisor delay before Q10 will conduct following detection of a pulsetransmitted from the implanted pacer. On the other hand, if R36 is setwith a comparatively high ohmic value, C11 will have a greater residualvoltage on it after conduction of transistors Q8 and Q9 and less timewill be required to recharge the capacitor C11 after this event. Thismeans that Q10 will conduct in a shorter time after a pulse which istransmitted from the implanted pacer is detected and a control pulsewill be transmitted sooner from coil L2 to implanted coil Ll, thusmaking the implanted pacer provide a stimulating pulse earlier.

Variable resistors R29 and R36 are preferably ganged as indicated by thebroken line 42 in order that hysteresis will be proportional to the highpulse rates or some programable function.

Many different operating modes are attainable with the combination ofthe standby pacer and the external rate control. The physician mayselect the desired mode depending on the type of control he wants forthe treatment or testing of the particular patient. Generally, modeselection will involve merely turning knobs on the external control toadjust the timing resistor such as R29 and the hysteresis resistor R36to get the desired external control pulse rate and hysteresis rate.

The most common mode is where the internal pacer has a predeterminedintrinsic pulse rate and a fixed hysteresis rate or period. The externalcontrol is set to a high pulse rate above that of the internal pacer andabove the external hysteresis rate. The external control then drives theinternal pacer at the high set pacing rate of the external control. If anatural beat occurs, the hysteresis in the external control is resetand, after expiration of the hysteresis period, an external controlpacing pulse will be delivered. Unless another natural pulse occurs, theexternal control will continue to generate pacing pulses at its high setrate. Internal pacer hysteresis will have no effect when the externalhysteresis, the internal pace rate and the set external pace rate areall faster than the internal hysteresis rate.

Another mode results from setting the external control pace pulse rateso that the internal pacing pulse rate falls between this setting andthe hysteresis rate of the external control. All rates are, of course,above the internal pacer hysteresis rate. Assume the external control isdelivering pulses at its high set rate. Assume that a natural heart wavethen occurs. This will reset the internal and external hysteresisperiods. But the external hysteresis period is shorter than the internalso the external control will deliver one heart stimulating controlpulse. If no natural heartwave occurs soon thereafter, the externalcontrol will provide ensuing stimulating pulses at its high set rate.Therefore, the internal pacer rate is overridden by the higher externalrate setting.

Another mode involves setting the external high rate below the internalpacer high rate and setting the external hysteresis period shorter thanthe internal pacer hysteresis period. In effect, this mode results inrelating the external controls hysteresis rate to the pacers intrinsicpulse rate. In this mode, if the internal pacer is stimulating at itshigh intrinsic rate and a natural heart wave occurs there will be asignal radiated from internal coil L1. This will reset the externalhysteresis period. At the end of this period, which is shorter than theinternal hysteresis period, if no natural beat occurs the externalcontrol will supply one stimulating pulse and then the internal pacerwill supply ensuing pulses at its intrinsic pace pulse rate. In thiscase the latter rate is higher than the pacing rate of the externalcontrol.

In another mode the internal hysteresis period is set shorter than theexternal hysteresis period and the internal pace rate is higher than theexternal. Occurrence of a natural heart wave will then set the shorterinternal hysteresis period and upon its expiration, the internal pacerwill supply the next and ensuing stimulating pulses at its high pacingrate. The external control will remain inhibited due to its hysteresisperiod never having a chance to expire. In this mode the externalcontrol is not redundant since it is always ready to provide anartificial stimulating pulse and would be used for emergency pacinguntil the internal pacer can be replaced such as when its battery may berunning down and occasionally missing a beat.

If the external controls pacing rate and hysteresis rate are set belowthe hysteresis and pacing rate of the internal pacer, the latter willremain inhibited but may be used as a standby to a standby implantedpacer.

To recapitulate operation of the external control shown in FIG. 2, R29is set for producing pulse rates different than but usually in excess ofthe pulse rates of the implanted pacer. The range is usually from abovepulses per minute to pulses per minute depending on the physiciansdesire and the paients needs. The high rate pulses are transmitted fromexternal control coil L2 to coil L1 in the implanted pacer by means ofwhich the rate of the implanted pacer is captured or dominated. Whenevera natural or an internally stimulated beat occurs, a pulse is detectedby input coil L3 and, after suitable processing, causes C1 1 to beprematurely discharged in which case the external hysteresis circuit isreset. If a natural beat does not occur at the end of the externalhysteresis period, the external control will provide a first stimulatingpulse and then a series of higher rate pulses as required.

FIG. 3 is an alternative form of external rate control. Parts in thisfigure which are similar to those in preceding figures are given thesame reference numerals. In this arrangement a regular external standbypacer 45 is adapted to serve as an external rate control for animplanted spacer 46. The traditional use of an external standby pacer isfor temporarily pacing the heart pending installation of an implantedunit or as a safety measure for patients with a cardiac problem. Theexternal pacer usually connects to the heart by means of a conductivecatheter, not shown, which leads from the output terminals 47 and 48through a blood vessel to the heart. Such pacer usually has a ratecontrol 49, a standby or fixed rate mode control 50 and a currentcontrol 51. This type of pacer operates similarly to a standby implantedpacer in the respect that it provides an artificial stimulating pulsewhenever the natural heartbeat is missed or delayed for a predeterminedperiod of time. In this embodiment, the leads 52 and 53 which wouldnormally run to the heart to supply artificial pulses and to pick upnatural waves, are bridged by a resistor R40 which is part of a voltagedivider that includes R41.

Coil L3, which detects transmitted pulses from implanted coil Ll uponoccurrence of a natural heart signal or internal pace pulses drives anamplifier 54. Output signals from amplifier 54 produce a voltage acrossR40 in a voltage divider R40 and R41 which is similar in magnitude to asignal that would be detected on the heart if the unit were used as apacer instead of an external control. Pace signals from the externalstandby control appear across R40 and trigger a coil driver 55 which iscomparable in function to driver transistors Q6 and Q12 in FIGS. 1 and2, respectively. In other words, each time a signal from L1 is detected,it is sensed by the external pacer unit 45 which is thereby inhibited.If no signal is detected in receiver coil L3, the external unit 45provides a signal over its leads 53 and 52 to coil driver 55 and coil L2which transmits a large amplitude electromagnetic pulse to the implantedpacer coil L1, causing the pacer to provide an artificial stimulatingpulse to the heart. In reality, then, amplifier 54 serves as a detectorfor internally generated signals that inhibits the external control 45from producing pacer triggering signals to coil driver 55 if there arenatural heart waves or internal pace pulses at a rate higher than theset pace rate or hysteresis rate of the external control.

The external rate controls shown in both FIGS. 2 and 3 have no effect onthe implanted pacer if the pulse rate of the external rate controls areset below the intrinsic and the hysteresis rate of the implanted pacer.

For the sake of illustrating the principles of the invention the abovedescription assumes that the remote rate control is situated external tothe body and that the standby pacer power supply is implanted in bodytissue and connected to the heart with relatively long conductors or anintravascular conductive catheter. However, the new rate control mayalso be adapted for implantation in body tissue remote from the pacer.It is only necessary to seal the control against permeation by bodyfluids and to provide suitable means for varying the pulse period of thecontrol which means can be operated from outside of the body. Forexample, instead of using a variable potentiometer or resistor such asR29 to change the pulse period, a resistor and magnetic reed switchnetwork could be used. The reed switches could be operated selectivelyby means of an external magnet so as to change the charging ordischarging time of the timing capacitor C11 and, hence, the pulseperiod. An adjustable potentiometer that is operable with a needle thatpenetrates the tissue can also be used to change pulse timing. Solidstate memory systems controlled by remotely operable switches or byinduction may also be incorporated in the implanted remote rate control.Various types of capacitor switching may be used too.

Advances in solid state technology and in power supplies are expected topermit such size reduction of standby pacers that it will soon bepractical to mount a pacer directly on the heart and to connect it tothe heart without using flexible leads. Long life power sources anddurable solid state circuitry is expected to result in infrequentreplacement and infrequent need to gain access to the pacer. On theother hand, the optimum pacing rate for a patient is likely to changeover such a long time. In such case, the new remote rate control can beimplanted beneath the skin surface in a readily accessible place remotefrom the pacer on the heart and the patients heart rate can be changedat will without surgery. The remote rate control would then bereplaceable, when required, by a superficial surgical procedure usinglocal anesthesia. Replacement of conductors leading to the heart wouldbe obviated.

In summary, two different types of remote rate controls for implantedstandby cardiac pacers have been described. Each requires that theimplanted pacer be modified to transmit an electromagnetic radiationpulse in response to the occurrence of a natural or artificiallystimulated heartbeat. If the signal is one which corresponds with anatural heartbeat, the remote control is inhibited during a hysteresisperiod after which an artificial stimulating pulse is supplied to theheart. The heart is always stimulated at a rate determined by the setrate of the remote control assuming the control rate is higher than theinternal pace rate. The hysteresis features of the internal standbypacer and the remote control are nonconflicting. Competition betweennatural and artificial heart stimuli is avoided.

Although embodiments of the invention have been described inconsiderable detail, such description is intended to be illustrativerather than limiting, for the invention may be variously embodied and itis to be limited only by interpretation of the claims which follow.

Iclaim:

l. A rate control for remote control of the pacing pulse rate of a heartpacer that cooperates with the remote control, which pacer ischaracterized by having an intrinsic pulse period and by radiating asignal from it each time a natural heart stimulus occurs or anartificially generated heart stimulus caused by it occurs and by havingits pulse period controlled by control signals which are radiated intoit from the remote rate control, said remote rate control comprising:

a. a pulse generator for producing control pulse signals with apredetermined period between them which is different than the intrinsicpacing pulse period of said pacer,

a first pulse timing means for controlling the pulse period of thecontrol pulse generator,

c. means for transmitting to said pacer signals corresponding with saidcontrol pulse signals,

d. means for detecting in said remote rate control signals that arereceived from said pacer which detected signals correspond withoccurrence of either a natural or an artificial heart stimulus signal,and

e. means for resetting the pulse timing means of the remote rate controlto begin a predetermined time period in response to occurrence of adetected signal from said cooperating pacer corresponding with either anatural heart signal or an artificially generated stimulus signal, saidtiming means then causing said control pulse generator to produce acontrol pulse if neither of said signals is detected before the end ofsaid reset time interval.

2. The invention set forth in claim 1 including:

a. hysteresis means for resetting said pulse timing means to produce atiming period which is different than said predetermined period inresponse to occurrence of a detected signal from said pacercorresponding with said either natural or artificial heart stimulussignals, said control pulse generator being controlled by said timingmeans to produce a control signal in correspondence with expiration ofsaid different timing period to cause said cooperating pacer toartificially stimulate the heart, said timing means then operating toproduce control signals to cause said pacer to artificially stimulatethe heart in correspondence with said predetermined period until anothersignal from said pacer is detected.

3. A rate control for remote control of the pacing pulse rate of a firststandby heart pacer that cooperates with the remote control, which paceris characterized by having an intrinsic pulse period and by radiating asignal each time a natural heart stimulus occurs or an artificiallystimulated heartbeat caused by it occurs and by having its pulse ratedominated to correspond with the rate at which control signals areradiated into it, said remote rate control comprising:

a. a second standby pacer which is adapted to remotely control saidfirst standby pacer which is implantable in the body,

b. said second standby pacer having output terminals and a pulsegenerator for selectively supplying control pulses to the outputterminals at a selected rate which is different than the intrinsic pulserate of said first implantable standby pacer, said second pacerincluding timing means controlling its pulse generator to produce pulsesat said selected rate and including means for inhibiting production of apulse in response to occurrence of a signal across said output terminalscorresponding with a signal from said first heart pacer,

c. means connected with said output terminals of said second standbypacer for receiving pulse signals which correspond with signals that aretransmitted through body tissue from said first implantable standbypacer and correspond with occurrence of a natural or stimulatedheartbeat,

(1. means for delivering signals corresponding with said receivedsignals to said second standby pacer output terminals to thereby actuatesaid inhibiting means to inhibit said second standby pacer untilexpiration of its pulse period, said received signals corresponding witheither a natural heart stimulus or an artificial stimulus caused by saidfirst pacer, and

e. signal transmitting means associated with said second standby pacerfor receiving signals from said second standby pacer when it is notinhibited and for transmitting corresponding control signals to saidfirst implantable pacer so as to control its pacing pulse period insynchronism with the period of the second standby pacer.

4. The invention set forth in claim 3 wherein:

:1. a resistor voltage divider means a part of which is connected acrosssaid second pacer output terminals,

b. said signal receiving means includes a receiving 5 coil,

c. an amplifier having its input connected to the receiving coil and itsoutput connected to another part of said resistor means which isconnected to said first part to produce a voltage across said first partof said resistor means when there is a detected signal, said voltageinhibiting said second pacer,

d. said signal transmitting means including a pulse amplifying means anda transmitting coil,

e. said pulse amplifying means being connected to said terminals of saidsecond pacer whereby to receive pulses from said second pacer when it isnot inhibited and to drive said transmitting means for controlling saidimplantable pacer.

5. An implantable heart pacer that is adapted for having its intrinsicheart pacing pulse rate altered in response to receipt of signals at theselected pulse rate of a remotely situated control, said pacercomprising:

a. a pacing pulse generator means that produces heart pacing pulses atan intrinsic rate and means for sensing a natural heart signal and meansresponsive to said sensing means for inhibiting said pacing pulsegenerator from producing a pacing pulse for a predetermined period uponoccurrence of a natural heart signal,

. timing means controlling the pacing pulse generator period,

c. means in said implantable pacer for receiving control signalsradiated from a remote rate control means at a rate that is differentthan the intrinsic pacer pulse rate and for radiating signals to saidremote control, said timing means including means responding to saidreceived signals by altering the period between intrinsic pace pulsesand changing the rate of said pacing pulse generator to cor respond withthe pulse rate of said remote control, and

d. means for producing a control signal in response to occurrence ofeither a natural heart signal or an artificial pacing signal andincluding means coupled thereto for radiating a remote controlinhibiting signal outside of the implantable pacer in correspondencewith said either signals.

6. The invention set forth in claim 5 wherein:

a. said timing means associated with the pacing pulse generator of thepacer includes timing resistor means and a timing capacitor connectedserially to each other, said timing capacitor being connected to saidpulse generator and the voltage on said timing capacitor determining thetime when said pacb. switching means having a control terminal and loadterminals the latter of which are connected in series with said pulsecapacitor and said coil means,

c. the control terminal of said switching means being connected toreceive last named control signals whereby to establish conductionacross its load terminals so as to discharge said capacitor through saidcoil, whereupon said coil radiates a heart stimulus indicating signal tosaid remote rate control.

8. A remote rate control for controlling the artificial heart stimuluspulse rate of an implantable pacer which is characterized by having anintrinsic pulse rate and by it radiating a signal to the outside of thebody upon occurrence of each natural heart stimulus and each artificialstimulus pulse that is produced by the pacer, said pacer being adaptedto have its pulse rate controlled in relation to the rate at whichcontrol signals are radiated into it, said remote rate controlcomprising:

a. pulse generating means,

b. signal radiating means for radiating signals to said implantablepacer in correspondence with pulses from said pulse generating means,

c. pulse timing means for controlling said pulse generating means toproduce pulses with a certain interpulse period,

d. detector means for detecting signals which are radiated from saidimplantable pacer in correspondence with occurrence of either a naturalheart stimulus or an artificially generated stimulus of said pacer, and

e. means responding to detection of either a natural heart stimulus oran artificial stimulus from said pacer by resetting said pulse timingmeans,

. said pulse timing means comprising a d-c source terminal and resistormeans and capacitor means serially connected to each other and connectedwith said source terminal, a predetermined voltage on said capacitorresulting from charging it from said source terminal controlling saidpulse generator to produce a pulse, and

. means connected with said capacitor means and responding to detectionof either of said stimuli by reducing the voltage on said capacitormeans to less than said predetermined voltage to thereby inhibit saidremote pulse generator means for a second predetermined period.

. The invention set forth in claim 8 including: semiconductor switchmeans connected to said capacitor for being rendered conductive by apredetermined voltage on said capacitor to cause said pulse generator toproduce a pulse and effect the delivery of a corresponding radiatedsignal,

b. a bias voltage divider means connected to said semiconductor switchmeans and applying a bias voltage thereto,

c. said means responding to said detection of either

1. A rate control for remote control of the pacing pulse rate of a heartpacer that cooperates with the remote control, which pacer ischaracterized by having an intrinsic pulse period and by radiating asignal from it each time a natural heart stimulus occurs or anartificially generated heart stimulus caused by it occurs and by havingits pulse period controlled by control signals which are radiated intoit from the remote rate control, said remote rate control comprising: a.a pulse generator for producing control pulse signals with apredetermined period between them which is different than the intrinsicpacing pulse period of said pacer, b. a first pulse timing means forcontrolling the pulse period of the control pulse generator, c. meansfor transmitting to said pacer signals corresponding with said controlpulse signals, d. means for detecting in said remote rate controlsignals that are received from said pacer which detected signalscorrespond with occurrence of either a natural or an artificial heartstimulus signal, and e. means for resetting the pulse timing means ofthe remote rate control to begin a predetermined time period in responseto occurrence of a detected signal from said cooperating pacercorresponding with either a natural heart signal or an artificiallygenerated stimulus signal, said timing means then causing said controlpulse generator to produce a control pulse if neither of said signals isdetected before the end of said reset time interval.
 1. A rate controlfor remote control of the pacing pulse rate of a heart pacer thatcooperates with the remote control, which pacer is characterized byhaving an intrinsic pulse period and by radiating a signal from it eachtime a natural heart stimulus occurs or an artificially generated heartstimulus caused by it occurs and by having its pulse period controlledby control signals which are radiated into it from the remote ratecontrol, said remote rate control comprising: a. a pulse generator forproducing control pulse signals with a predetermined period between themwhich is different than the intrinsic pacing pulse period of said pacer,b. a first pulse timing means for controlling the pulse period of thecontrol pulse generator, c. means for transmitting to said pacer signalscorresponding with said control pulse signals, d. means for detecting insaid remote rate control signals that are received from said pacer whichdetected signals correspond with occurrence of either a natural or anartificial heart stimulus signal, and e. means for resetting the pulsetiming means of the remote rate control to begin a predetermined timeperiod in response to occurrence of a detected signal from saidcooperating pacer corresponding with either a natural heart signal or anartificially generated stimulus signal, said timing means then causingsaid control pulse generator to produce a control pulse if neither ofsaid signals is detected before the end of said reset time interval. 2.The invention set forth in claim 1 including: a. hysteresis means forresetting said pulse timing means to produce a timing period which isdifferent than said predetermined period in response to occurrence of adetected signal from said pacer corresponding with said either naturalor artificial heart stimulus signals, said control pulse generator beingcontrolled by said timing means to produce a control signal incorrespondence with expiration of said different timing period to causesaid cooperating pacer to artificially stimulate the heart, said timingmeans then operating to produce control signals to cause said pacer toartificially stimulate the heart in correspondence with saidpredetermined period until another signal from said pacer is detected.3. A rate control for remote control of the pacing pulse rate of a firststandby heart pacer that cooperates with the remote control, which paceris characterized by having an intrinsic pulse period and by radiating asignal each time a natural heart stimulus occurs or an artificiallystimulated heartbeat caused by it occurs and by having its pulse ratedominated to correspond with the rate at which control signals areradiated into it, said remote rate control comprising: a. a secondstandby pacer which is adapted to remotely control said first standbypacer which is implantable in the body, b. said second standby pacerhaving output terminals and a pulse generator for selectively supplyingcontrol pulses to the output terminals at a selected rate which isdifferent than the intrinsic pulse rate of said first implantablestandby pacer, said second pacer including timing means controlling itspulse generator to produce pulses at said selected rate and includingmeans for inhibiting production of a pulse in response to occurrence ofa signal across said output terminals corresponding with a signal fromsaid first heart pacer, c. means connected with said output terminals ofsaid second standby pacer for receiving pulse signals which correspondwith signals that are transmitted through body tissue from said firstimplantable standby pacer and correspond with occurrence of a natural orstimulated heartbeat, d. means for delivering signals corresponding withsaid received signals to said second standby pacer output terminals tothereby actuate said inhibiting means to inhibit said second standbypacer until expiration of its pulse period, said received signalscorresponding with either a natural heart stimulus or an artificialstimulus caused by said first pacer, and e. signal transmitting meansassociated with said second standby pacer for receiving signals fromsaid second standby pacer when it is not inhibited and for transmittingcorresponding control signals to said first implantable pacer so as tocontrol its pacing pulse period in synchronism with the period of thesecond standby pacer.
 4. The invention set forth in claim 3 wherein: a.a resistor voltage divider means a part of which is connected acrosssaid second pacer output terminals, b. said signal receiving meansincludes a receiving coil, c. an amplifier having its input connected tothe receiving coil and its output connected to another part of saidresistor means which is connected to said first part to produce avoltage across said first part of said resistor means when there is adetected signal, said voltage inhibiting said second pacer, d. saidsignal transmitting means including a pulse amplifying means and atransmitting coil, e. said pulse amplifying means being connected tosaid terminals of said second pacer whereby to receive pulses from saidsecond pacer when it is not inhibited and to drive said transmittingmeans for controlling said implantable pacer.
 5. An implantable heartpacer that is adapted for having its intrinsic heart pacing pulse ratealtered in response to receipt of signals at the selected pulse rate ofa remotely situated control, said pacer comprising: a. a pacing pulsegenerator means that produces heart pacing pulses at an intrinsic rateand means for sensing a natural heart signal and means responsive tosaid sensing means for inhibiting said pacing pulse generator fromproducing a pacing pulse for a predetermined period upon occurrence of anatural heart signal, b. timing means controlling the pacing pulsegenerator period, c. means in said implantable pacer for receivingcontrol signals radiated from a remote rate control means at a rate thatis different than the intrinsic pacer pulse rate and for radiatingsignals to said remote control, said timing means inclUding meansresponding to said received signals by altering the period betweenintrinsic pace pulses and changing the rate of said pacing pulsegenerator to correspond with the pulse rate of said remote control, andd. means for producing a control signal in response to occurrence ofeither a natural heart signal or an artificial pacing signal andincluding means coupled thereto for radiating a remote controlinhibiting signal outside of the implantable pacer in correspondencewith said either signals.
 6. The invention set forth in claim 5 wherein:a. said timing means associated with the pacing pulse generator of thepacer includes timing resistor means and a timing capacitor connectedserially to each other, said timing capacitor being connected to saidpulse generator and the voltage on said timing capacitor determining thetime when said pacing pulse generator will pulse, b. saidelectromagnetic means comprising a coil connected with said timingcapacitor and across which is developed a voltage when a signal isreceived which voltage is additive to that on the timing capacitor,whereby said pacing pulse generator produces a pulse at a time beforeexpiration of its intrinsic interpulse interval.
 7. The invention setforth in claim 5 including: a. a pulse capacitor connected to saidradiating means which is a coil means, and a charging circuit for saidpulse capacitor, b. switching means having a control terminal and loadterminals the latter of which are connected in series with said pulsecapacitor and said coil means, c. the control terminal of said switchingmeans being connected to receive last named control signals whereby toestablish conduction across its load terminals so as to discharge saidcapacitor through said coil, whereupon said coil radiates a heartstimulus indicating signal to said remote rate control.
 8. A remote ratecontrol for controlling the artificial heart stimulus pulse rate of animplantable pacer which is characterized by having an intrinsic pulserate and by it radiating a signal to the outside of the body uponoccurrence of each natural heart stimulus and each artificial stimuluspulse that is produced by the pacer, said pacer being adapted to haveits pulse rate controlled in relation to the rate at which controlsignals are radiated into it, said remote rate control comprising: a.pulse generating means, b. signal radiating means for radiating signalsto said implantable pacer in correspondence with pulses from said pulsegenerating means, c. pulse timing means for controlling said pulsegenerating means to produce pulses with a certain interpulse period, d.detector means for detecting signals which are radiated from saidimplantable pacer in correspondence with occurrence of either a naturalheart stimulus or an artificially generated stimulus of said pacer, ande. means responding to detection of either a natural heart stimulus oran artificial stimulus from said pacer by resetting said pulse timingmeans, f. said pulse timing means comprising a d-c source terminal andresistor means and capacitor means serially connected to each other andconnected with said source terminal, a predetermined voltage on saidcapacitor resulting from charging it from said source terminalcontrolling said pulse generator to produce a pulse, and g. meansconnected with said capacitor means and responding to detection ofeither of said stimuli by reducing the voltage on said capacitor meansto less than said predetermined voltage to thereby inhibit said remotepulse generator means for a second predetermined period.